1. Field of Invention
The present invention relates to semiconductor light emitting devices including light extracting structures.
2. Description of Related Art
Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.
FIG. 1 illustrates a composite growth substrate, described in more detail in US 2007/0072324, which is incorporated herein by reference. “Substrate 10 includes a host substrate 12, a seed layer 16, and a bonding layer 14 that bonds host 12 to seed 16 . . . [T]he layers in substrate 10 are formed from materials that can withstand the processing conditions required to grow the semiconductor layers in the device. For example, in the case of a III-nitride device grown by MOCVD, each of the layers in substrate 10 must be able to tolerate an H2 ambient at temperatures in excess of 1000° C.; in the case of a III-nitride device grown by MBE, each of the layers in substrate 10 must be able to tolerate temperatures in excess of 600° C. in a vacuum.
“Host substrate 12 provides mechanical support to substrate 10 and to the semiconductor device layers 18 grown over substrate 10. Host substrate 12 is generally between 3 and 500 microns thick and is often thicker than 100 microns. In embodiments where host substrate 12 remains part of the device, host substrate 12 may be at least partially transparent if light is extracted from the device through host substrate 12. Host substrate 12 generally does not need to be a single crystal material since device layers 18 are not grown directly on host substrate 12. In some embodiments, the material of host substrate 12 is selected to have a coefficient of thermal expansion (CTE) that matches the CTE of device layers 18 and the CTE of seed layer 16. Any material able to withstand the processing conditions of epitaxial layers 18 may be suitable . . . including semiconductors, ceramics, and metals. Materials such as GaAs which have a CTE desirably close to the CTE of device layers 18 but which decompose through sublimation at the temperatures required to grow III-nitride layers by MOCVD may be used with an impermeable cap layer such as silicon nitride deposited between the GaAs host and seed layer 16.
“Seed layer 16 is the layer on which device layers 18 are grown, thus it must be a material on which III-nitride crystal can nucleate. Seed layer 16 may be between about 50 Å and 1 μm thick. In some embodiments seed layer 16 is CTE-matched to the material of device layers 18. Seed layer 16 is generally a single crystal material that is a reasonably close lattice-match to device layers 18. Often the crystallographic orientation of the top surface of seed layer 16 on which device layers 18 are grown is the wurtzite [0001] c-axis. In embodiments where seed layer 16 remains part of the finished device, seed layer 16 may be transparent or thin if light is extracted from the device through seed layer 16.
“One or more bonding layers 14 bond host substrate 12 to seed layer 16. Bonding layer 14 may be between about 100 Å and 1 μm thick. Examples of suitable bonding layers including SiOx such as SiO2, SiNx such as Si3N4, HfO2, mixtures thereof, metals such as Mo, Ti, TiN, other alloys, and other semiconductors or dielectrics. Since bonding layer 14 connects host substrate 12 to seed layer 16, the material forming bonding layer 14 is selected to provide good adhesion between host 12 and seed 16. In some embodiments, bonding layer 14 is a release layer formed of a material that can be etched by an etch that does not attack device layers 18, thereby releasing device layers 18 and seed layer 16 from host substrate 12. For example, bonding layer 14 may be SiO2 which may be wet-etched by HF without causing damage to III-nitride device layers 18. In embodiments where bonding layer 14 remains part of the finished device, bonding layer 14 is preferably transparent or very thin. In some embodiments, bonding layer 14 may be omitted, and seed layer 16 may be adhered directly to host substrate 12.
“Further strain relief in epitaxial layers 18 may be provided by forming the seed layer as stripes or a grid over bonding layer 14, rather than as a single uninterrupted layer. Alternatively, seed layer may be formed as a single uninterrupted layer, then removed in places, for example by forming trenches, to provide strain relief. A single uninterrupted seed layer 16 may be attached to host substrate 12 through bonding layer 14, then patterned by conventional lithography techniques to remove portions of the seed layer to form stripes. The edges of each of the seed layer stripes may provide additional strain relief by concentrating dislocations within epitaxial layers 18 at the edges of the stripes of seed layer. The composition of seed layer 16, bonding layer 14, and the nucleation layer may be selected such that the nucleation layer material nucleates preferentially on seed layer 16, not on the portions of bonding layer 14 exposed by the spaces between the portions of seed layer 16.
“On a wafer of light emitting devices, the trenches in seed layer 16 . . . may be spaced on the order of a single device width, for example, hundred of microns or millimeters apart. A wafer of devices formed on a composite substrate with a patterned seed layer may be divided such that the edges of the seed layer portions are not located beneath the light emitting layer of individual devices, since the dislocations concentrated at the edge of the seed layers may cause poor performance or reliability problems. Alternatively, multiple trenches may be formed within the width of a single device, for example, spaced on the order of microns or tens of microns apart. Growth conditions on such substrates may be selected such that the nucleation layer formed over seed layer 16, or a later epitaxial layer, coalesces over the trenches formed in seed layer 16, such that the light emitting layer of the devices on the wafer is formed as a continuous layer uninterrupted by the trenches in seed layer 16.
When the seed layer is a III-nitride material, “the seed layer is grown strained on the growth substrate. When the seed layer 16 is connected to host substrate 12 and released from the growth substrate, if the connection between seed layer 16 and host substrate 16 is compliant, for example a compliant bonding layer 14, seed layer 16 may at least partially relax. Thus, though the seed layer is grown as a strained layer, the composition may be selected such that the lattice constant of the seed layer, after the seed layer is released from the growth substrate and relaxes, is reasonably close or matched to the lattice constant of the epitaxial layers 18 grown over the seed layer.
“For example, when a III-nitride device is conventionally grown on Al2O3, the first layer grown on the substrate is generally a GaN buffer layer with an a lattice constant of about 3.19. The GaN buffer layer sets the lattice constant for all of the device layers grown over the buffer layer, including the light emitting layer which is often InGaN. Since relaxed, free standing InGaN has a larger a lattice constant than GaN, the light emitting layer is strained when grown over a GaN buffer layer. In contrast, . . . an InGaN seed layer may be grown strained on a conventional substrate, then bonded to a host and released from the growth substrate such that the InGaN seed layer at least partially relaxes. After relaxing, the InGaN seed layer has a larger a lattice constant than GaN. As such, the lattice constant of the InGaN seed layer is a closer match than GaN to the lattice constant of a relaxed free standing layer of the same composition as the InGaN light emitting layer. The device layers grown over the InGaN seed layer, including the InGaN light emitting layer, will replicate the lattice constant of the InGaN seed layer. Accordingly, an InGaN light emitting layer with a relaxed InGaN seed layer lattice constant is less strained than an InGaN light emitting layer with a GaN buffer layer lattice constant. Reducing the strain in the light emitting layer may improve the performance of the device.
“III-nitride seed layer materials may require additional bonding steps in order to form a composite substrate with a III-nitride seed layer in a desired orientation. III-nitride layers grown on sapphire or SiC growth substrates are typically grown as c-plane wurtzite. Such wurtzite III-nitride structures have a gallium face and a nitrogen face. III-nitrides preferentially grow such that the top surface of the grown layer is the gallium face, while the bottom surface (the surface adjacent to the growth substrate) is the nitrogen face. Simply growing seed layer material conventionally on sapphire or SiC then connecting the seed layer material to a host and removing the growth substrate would result in a composite substrate with a III-nitride seed layer with the nitrogen face exposed. As described above, III-nitrides preferentially grow on the gallium face, i.e. with the gallium face as the top surface, thus growth on the nitrogen face may undesirably introduce defects into the crystal, or result in poor quality material as the crystal orientation switches from an orientation with the nitrogen face as the top surface to an orientation with the gallium face as the top surface.
“To form a composite substrate with a III-nitride seed layer with the gallium face as the top surface, seed layer material may be grown conventionally on a growth substrate, then bonded to any suitable first host substrate, then separated from the growth substrate, such that the seed layer material is bonded to the first host substrate through the gallium face, leaving the nitrogen face exposed by removal of the growth substrate. The nitrogen face of the seed layer material is then bonded to a second host substrate 10, the host substrate of the composite substrate . . . . After bonding to the second host substrate, the first host substrate is removed by a technique appropriate to the growth substrate. In the final composite substrate, the nitrogen face of the seed layer material 16 is bonded to host substrate 12 (the second host substrate) through optional bonding layer 14, such that the gallium face of III-nitride seed layer 16 is exposed for growth of epitaxial layers 18.
“For example, a GaN buffer layer is conventionally grown on a sapphire substrate, followed by an InGaN layer which will form the seed layer of a composite substrate. The InGaN layer is bonded to a first host substrate with or without a bonding layer. The sapphire growth substrate is removed by laser melting of the GaN buffer layer adjacent to the sapphire, then the remaining GaN buffer layer exposed by removing the sapphire is removed by etching, resulting in an InGaN layer bonded to a first host substrate. The InGaN layer may be implanted with a material such as hydrogen, deuterium, or helium to form a bubble layer at a depth corresponding to the desired thickness of the seed layer in the final composite substrate . . . . The InGaN layer may optionally be processed to form a surface sufficiently flat for bonding. The InGaN layer is then bonded with or without a bonding layer to a second host substrate, which will form the host in the final composite substrate. The first host substrate, InGaN layer, and second host substrate are then heated as described above, causing the bubble layer implanted in the InGaN layer to expand, delaminating the thin seed layer portion of the InGaN layer from the rest of the InGaN layer and the first host substrate, resulting in a finished composite substrate as described above with an InGaN seed layer bonded to a host substrate.”